In magnetic record carriers which execute a movement relative to the magnetic heads, as in disk storages, devices are known for the servo track following for the purpose of controlling and maintaining the track position of magnetic heads. Servo position signals are recorded in offset fashion on a magnetic record carrier on both sides of the middle of recording tracks, said signals being wider than the track. These servo position signals are pre-coded information signals which can be located in so-called servo sectors, i.e. individual sectors in a circumferential data track which serve for servo purposes only, and which can be alternatingly interrupted by longer data sectors within one track. However, the servo position signals can also be recorded on separate servo tracks, or on a separate side of a disk of a disk pack.
The servo position signals on both sides of the tracks are read and detected by the magnetic heads. A position error signal supplying data on the deviation of the magnetic head from the track center is detected by forming the difference of the two servo position signals to the right and left of the middle of the track. If this difference is zero, it is assumed that the head is precisely over the track. A device for servo track following control as described above is shown in U.S. Pat. No. 3,185,972, issued May 25, 1965.
The signal induced in the magnetic head is exponentially inversely proportional to the distance between the magnetic surface of the record carrier and the head gap. If there is a difference of head gap distance relative to the record carrier, the servo position signals induce signals of different intensity in the magnetic head, even when the magnetic head is positioned precisely over the middle of the track. Owing to this tilting, which appears particularly in disk storages with flexible disks, because the structure of the substrate, the roughness of the surface, the form and suspension of the magnetic head, do not always ensure that the head flies over the entire track width in parallel to the disk surface, a position error signal is generated which wrongly differs from zero and thus causes a setting of the magnetic head by the following control circuit and the access mechanism. Consequently, the head is shifted although it had been neither necessary nor justified. A tilting of the magnetic head thus simulates a radial positional deviation, with the consequence that the control circuit reacts with a deviation from the middle of the track.
It is therefore an object of the present invention to design a device in such a manner that even deviations from the uniform height between magnetic head and track, or servo signals respectively, do not cause a wrong reaction, or in other words, that magnetic head tiltings or oblique record carriers are detected and corrected in such a manner that the head follows the track as required.
The providing of the servo position signal encoding in accordance with the invention, and the kind of evaluation of the auxiliary servo position signals advantageously used for normalizing the position error signal actuating the control, for synchronizing and for improving the signal-to-noise ratio, e.g. in the switching in addition to the standard or main servo position signals.
Another problem appearing in connection with servo track follower circuits are the so-called uniform errors or disturbance variables. Under ideal conditions, these control circuits keep the middle of the magnetic head precisely over the middle of the track. In reality, however, the middle of the head does not move exactly on the middle of the track. One of the reasons is the noise, and consequently a lack of precision, during the determination of the position. Another important reason for errors is the disturbance variables acting on the circuit following the control. Examples for such disturbance variables are aerodynamic forces acting on the magnetic head and displacing it to the track side, as well as offset voltages in amplifiers. Without specific countermeasures in the control circuit, such phenomena cause stationary consequential errors, or uniform errors, respectively.
It is generally known, as shown by an example in U.S. Pat. No. 3,534,344, issued Oct. 13, 1970, to eliminate such uniform disturbance variables by means of controls of an integral contribution. However, the integral contribution considerably restricts the dynamics of the control circuit, and transient processes are prolonged. For these reasons, such controls are quite unsuitable for servo track controls of magnetic heads which have to react very quickly.
In control systems of servo track following controls it is known, as shown for example in U.S. Pat. No. 3,725,764, issued Apr. 3, 1973, that the control system shows delayed action which is to be compensated. For this compensation, it describes a method of generating an additional signal that is proportional to the deviation velocity, and adding it to the deviation signal compensated in phase position and amplitude. This sum signal is applied via another compensator to the control element as a control signal. For the compensation of uniform disturbance variables, such a control is not suitable as it does not respond, or only incorrectly, to this type of disturbance variable.
A method known from U.S. Pat. No. 3,881,184, issued Apr. 29, 1975, compensates periodically recurrent errors within servo track following controls in that the periodically recurrent errors are first detected, then stored and subsequently added to the position error signal in a manner according to their position. This method for eliminating periodic errors does not give any indication with respect to solving the problem of uniform disturbance variables and their elimination.
With respect to this above mentioned problem of uniform errors or disturbance variables, an object of the present invention furthermore consists in compensating these disturbance variables in control circuits as in particular in servo track following controls simply, reliably, and without further influencing the characteristic of the control circuit.
Connected with this solution is the important advantage that uniform disturbance variables are simply compensated in control circuits without the control circuit dynamics being affected, and that consequently, if used in servo track following controls for magnetic heads, the latter are kept in the middle of the track.